HF radio transmission
60 2110 SSB Transceiver Getting Started Guide
Sky wave
The sky wave is the most important form of HF propagation.
The radio wave is transmitted toward the sky and is reflected
by the ionosphere to a distant receiver on earth.
The reflective properties of the ionosphere change throughout
the day, from season to season, and yearly.
Figure 7: The reflective properties of the ionosphere
Frequency, distance and time of day
The extent to which a radio wave is reflected depends on the
frequency that is used. If the frequency is too low, the signal is
absorbed by the ionosphere. If the frequency is too high, the
signal passes straight through the ionosphere. Within the HF
band, low frequencies are generally considered to be in the
range of 2 to 10 MHz. High frequencies are above 10 MHz.
A frequency chosen for daytime transmission may not
necessarily be suitable for night-time use. During the day, the
layers of the ionosphere are thick. The layers absorb lower
frequencies and reflect higher frequencies. At night, the
ionosphere becomes very thin. The low frequencies that were
absorbed during the day are reflected and the high frequencies
that were reflected during the day pass straight through.
Summer HF communications usually operate on higher
frequencies than those used in winter over the same distance.
ionosphere
transmitter
receiver
emitted
HF wave
reflected
HF wave
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